In such carriages the floor height of the zone that is opposite a door of the vehicle is generally lowered in order to bring it as close as possible to the height of the station platform and thus avoid the presence of high steps, which render access to the railway vehicle difficult, especially for a wheelchair. The zone of reduced floor height is connected to the other zones of the carriage, which are of standard height, by ramps, which again make it possible to avoid the use of steps, which are particularly awkward for passengers with reduced mobility.
However, the heights of different station platforms are not constant and can be higher or lower depending on the location. Accordingly, even for a carriage which has been adapted to accommodate passengers with reduced mobility, it is often necessary to use an external removable ramp which connects the low zone of the carriage to the station platform in order to allow a wheelchair to disembark. Such ramps are complicated to use because they require the intervention of the train or station staff, who must be warned in advance and who must install the ramp quickly while the train is stopped in the station, often for only a short time.
In addition, there is generally not the space necessary to provide a functional space for housing equipment of the carriage beneath the low platform, so that the carriage becomes more bulky as a result.